Horford scored 24 points to help Atlanta rally for a 104-99 victory at Milwaukee on Sunday.
Smith added 23 points and nine rebounds for the Hawks, who trailed 98-95 with 1:05 remaining. Devin Harris added 16 points and Jeff Teague finished with 14.
"It shows a lot of toughness from our team," Horford said. "It is important, obviously. They are in the hunt, like we are. We want to position ourselves in the best scenario that we can. We've done the job."
The Hawks clinched the season series as they try to move up in the Eastern Conference standings. The Hawks are currently in fifth place and would open the playoffs on the road.
Smith converted a layup with 33.8 seconds to tie it at 99. After the Bucks turned it over, Dahntay Jones was fouled. He made the first and missed the second, but Anthony Tolliver grabbed the rebound for Atlanta and called timeout.
Hawks coach Larry Drew called Tolliver's rebound the play of the game.
"Playing on the road, you have to bring a certain mentality and I thought we did that tonight," he said. "We did enough to come out with a win."
Horford was fouled with 20.1 seconds remaining and hit both free throws to give the Hawks a 102-99 lead. Monta Ellis missed a 3-pointer for Milwaukee, and Teague made two foul shots to put the game away.
Ellis scored 20 points for Milwaukee, and Ersan Ilyasova added 19 points and 10 rebounds. Ilyasova was back in the lineup after missing the last three games with left hip pain.
Ilyasova said the loss was another missed opportunity for the Bucks, who are trying to move up from the eighth spot and a potential first-round playoff matchup with the Miami Heat.
"We were right there, but just had some bad turnovers at the end," he said. "We are a good team and I know we can still make a run."
Brandon Jennings, who was 4 for 15 from the field in another lackluster shooting performance, said time was running out on the Bucks to move up in the standings. Milwaukee has a tough upcoming schedule with games this week against the Los Angeles Lakers and the Oklahoma City Thunder.
"We have a lot of work to do," Jennings said. "We'll just see what happens."
Ellis led a Bucks comeback midway through the fourth quarter, making three 3-pointers to give Milwaukee a 97-92 lead with 2:45 remaining. But the Bucks couldn't hold off the Hawks.
"We didn't get discouraged on the late surge by Monta Ellis," Smith said. "We stayed focused and kept playing together. We both understand what is at stake. We just made a little bit more plays than they did."
Horford scored on a short jumper with 1 minute remaining to cut Milwaukee's lead to 98-97. Ellis missed a 3-pointer, but Larry Sanders was fouled going for the rebound. He hit one of two free throws to make it 99-97 with 44.1 seconds remaining, setting the stage for Atlanta's final rally.
Harris went to the locker room midway through the fourth quarter with an injured left foot and did not return. Drew said the Hawks would evaluate Harris on Monday.
Bucks F Luc Richard Mbah a Moute missed his eighth straight game with a sprained toe. Coach Jim Boylan said Mbah a Moute was improving, but he did not know when he would return. ... Sanders had 12 rebounds to extend his double-digit streak to eight, the longest in his career. ... With his 3-pointer with 2:41 remaining in the first quarter, Atlanta guard Kyle Korver has hit at least one 3 in 63 straight games, fifth longest in NBA history. The longest streak belongs to Dana Barros at 89 games.
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MILWAUKEE, WI - MARCH 24: Brandon Jennings #3 of the Milwaukee Bucks shoots against Jeff Teague #0 of the Atlanta...
MVP: Al Horford was great all day with 24 points, 7 rebounds, 6 assists and 2 steals. His most important play came when he hit two late free throws to make it a two-possession game.
X factor: Anthony Tolliver's steal on Monta Ellis' errant pass and his rebound on a missed free throw late in the game sealed the Hawks' comeback win.
That was ... atypical: The Hawks were able to withstand 11 fourth-quarter points from Ellis and ended the game with a 12-2 run, including some clutch free throw shooting which hasn't been their strong suit this season.